be very careful! many of the fish sold in pet stores grow to many times larger than the size they are sold at. I saw a blue regal tang that was well under 2 inches in a tank at my LFS.

I think that it should be a requirement by law for all Fish stores to post the:

Name of the Fish.
Full size of fish when it reaches matureity.
Lifespan of Fish in captivity.
Food Requirements for the fish.
Minimum Space requirements for the fish.
Good and bad tankmates for the particular fish.

I think this would greatly help people out and avoid alot of dead fish and impulse buys.

There is no hard fast answer to this. For example I would say a minimum size for a yellow tang is 55 gal. I would also not recommend having more than 1 of them in a 55 gal tank. Where as with other fish the minimum tank size might be 10 or 20 gal and depending onthe fish the maximum # of that species in a tank of the minimum size could be larger.

As above can you give us more info on what you want to put in the tank and what is already in the tank as well as the size of tank.